A galaxy is a collection of stars held together by their own gravity and dark matter; we live in the Milky Way galaxy, a spiral disk in space coiling it’s arms outwards. Between 10-14 billion years old, we are believed to be one of hundreds of billions in the universe, although so far we have only identified around ten thousands. The astronomer Hubble classified different types of galaxies in 1925:
There are two types of spiral galaxies, Sa-d,m for regular spirals and SBa-d,m for those with a bar in the spiral arms. They are a disk in space with a bulge at their centre and two arms spiralling away from this. They rotate slowly, the Milky Way takes 250 million years to compete a cycle. Inside the disk is gas, showing star formation is ongoing and is young compared to other galaxies. Their shapes are delicate and are found in the low density galactic field so gravity from other galaxies cannot harm them.
A bright light in the sky, elliptical galaxies are uniformly shaped, smooth with no distinguishing features and a similar bulge to spirals. Elliptical’s are large possibly containing trillions of stars; they are the oldest galaxies without the presence of gas or new stars and reside in the high density galactic field. Collision with other galaxies haven given them their shape by destroying the spiral arms.
Small galaxies, the Dwarf’s contain approximately a few billion stars compared to our 200-400 billion in the Milky Way. The Canis Minor Dwarf galaxy is our closest neighbour and has become a part of the Milk Way when it was caught in our gravity. An example of a Dwarf galaxy is a large Magellanic cloud, its mass is too low to repel other gravities. The smallest are called Ultra Compact Dwarf galaxies and can be only two hundred light years across.
These galaxies have no spiral arms but do possess a bulge and disk shape; somewhere in between the ages of Elliptical and Spiral galaxies, they contain no gas and no new star formation.
Small galaxies with no defined shape are classified as irregulars. They can be any age and have lost their form due to collisions with other galaxies; Irr-I galaxies have some structure remaining while Irr-II have none. The Cigar galaxy in Ursa Major is a famous irregular galaxy for its high formation of stars; it is brighter than the Milky Way.
Resembling the rim of a sombrero hat, it is a spiral galaxy without the inside bar and viewed sideways; with a bright bulge at the centre, a dust lane encompasses this and contains mainly hydrogen along with the dust for star creation.
There are many galaxies yet to be discovered and within these may come many more types and classifications that we have not yet imagined.